Method of adjusting power for a wavelength-division multiplexed optical transmission system

ABSTRACT

A method of adjusting power for a WDM optical transmission system comprising emitter means, an optical transmission line, and receiver means, the method being characterized in that for each wavelength channel, the power emitted for said channel by the emitter means is adjusted as a function of the optical powers received for said channel at a plurality of points distributed along the transmission line.

[0001] The present invention relates to a method of adjusting power for a wavelength-division multiplexed (WDM) optical transmission system.

[0002] Digital optical channel transmission, in particular over an amplified link, is limited firstly at low powers by noise and secondly at high powers by non-linear effects.

[0003] In any amplified link, there exists an optimum outlet power level from each of the amplifiers which ensures the lowest possible error rate.

[0004] For transmission without wavelength-division multiplexing, the optimum power can be adjusted by the power of the pump in each amplifier.

[0005] However, when the transmission uses wavelength-division multiplexing, it is no longer possible to adjust the gain of each of the signals individually, since the response of the amplifiers is not flat with varying wavelength.

[0006] Three main techniques have been described for resolving this problem.

[0007] One of those techniques consists in optimizing the dopants of the amplifying fiber and in optimizing mean population inversion: flatness of about 1.5 decibels (dB) over 30 nanometers (nm) can thus be obtained for gain of 25 dB.

[0008] In this respect, reference can be made to the following publication:

[0009] K. Inoue, T. Kominato, H. Toba, IEEE Photon. Techn. Letters 3, 718 (1991);

[0010] Sulhoff, Smart, Zyskind, Nagel, DiGiovanni, “Gain peaking in concatenated 980-nm-pumped EDFAs”, OFC'94, p. 40 (1994).

[0011] Another technique consists in using optical filters.

[0012] In general, those first two techniques are used for the purpose of limiting variations in gain to a few tenths of a decibel.

[0013] A third technique consists in pre-emphasizing the signals, i.e. in reducing inlet power at those wavelengths that have higher gain and in increasing inlet power of signals for which gain is lower. Pre-emphasis is considered to be optimized when the signal-to-noise ratios in the various channels are equal.

[0014] A technique of that type is described in particular in:

[0015] Chaplyvy, Nagel, Tkach, “Equalization in amplified WDM lightwave transmission systems”, IEEE Photon. Techn. Lett. 4, 8 (1992);

[0016] Chraplyvy, Tkach, Reichmann, Magill, Nagel, “End-to-end equalization experiments in amplified WDM lightwave systems”, IEEE Photon, Techn. Lett. 4, 428 (1993).

[0017] Nevertheless, that technique cannot correct cumulative gain differences of more than 25 dB. In addition, it increases the non-linear penalty for given signal-to-noise ratio.

[0018] Furthermore, one of the main sources of degradation in a WDM terrestrial transmission system having amplifiers with equalized gain is the way the gain characteristic curve varies as a function of temperature. This variation is about 1 dB per 25 dB of gain over a temperature range of 50° C. Variation in line losses is another factor that degrades the flatness of amplifier gain.

[0019] To resolve that problem, proposals have been made, in particular in European patent application No. 0 580 497, for a device which adjusts the source parameters as a function of the error rate on reception, as estimated or as measured.

[0020] Nevertheless, information concerning error rate is not always available on reception.

[0021] Proposals have already been made, in particular in Japanese patent abstract No. JP 08 223136 for techniques which consist in equalizing gains at each of the amplifiers in a transmission line.

[0022] The invention proposes a method which makes it simple to equalize continuously the transmission performance of the various wavelength division multiplexed channels.

[0023] More particularly, the invention provides a method of adjusting power for a WDM optical transmission system comprising emitter means, an optical transmission line, and receiver means, the method being characterized in that for each wavelength channel, the power emitted for said channel by the emitter means is adjusted as a function of the optical powers received for said channel at a plurality of points distributed along the transmission line.

[0024] Such a method is advantageously associated with the various following characteristics taken singly or in any technically feasible combination:

[0025] the various points distributed along the transmission line are the inlets to amplifiers distributed along said line;

[0026] for each channel, a reference power is continuously determined as a function of the optical powers received for said channel at a plurality of points distributed along the transmission line, said reference power being the power that ought to be received for said channel at one of the points along the transmission line, and the emitter means are controlled so as to servo-control the power at said point on said reference power;

[0027] said point is the inlet to the first amplifier;

[0028] for each channel i, a parameter f_(i) is determined which is a function of the powers received at the various points of the transmission line, and the reference power is modified by replacing its preceding value with a reference value that is a function of said preceding value and also of the parameter f_(i);

[0029] the new reference power is the product of the preceding reference power and a function of the parameter f_(i);

[0030] the parameter f_(i) is compared with the mean {overscore (f)} of the parameters f_(i) as determined for the various channels;

[0031] the parameter f_(i) is such that: $f_{i} = {\sum\limits_{{j = 1},m}P_{ij}^{- 1}}$

[0032] where P_(ij) represents the power at the jth point of the transmission line for channel i and where m is the number of points of said line; and

[0033] the new reference power is the product of the preceding reference value and a product η·f_(i), where the parameter η is selected to avoid modifying the total power of the signals on emission.

[0034] The invention also provides an optical transmission system, in particular of return-to-zero (RZ) type pulses, characterized in that it includes means for implementing such a method.

[0035] Other characteristics and advantages of the invention appear further from the following description which is purely illustrative and non-limiting, and which can be read with reference to the accompanying drawing in which FIG. 1 is a diagram of an optical transmission system in which a method of the invention can be implemented.

[0036] The transmission system shown in the figure comprises emitter means 1, receiver means 2, and a transmission line 3 which, for example, is a terrestrial transmission line and extends between said emitter and receiver means 1 and 2.

[0037] The emitter means 1 and the receiver means 2 include multiplexing and demultiplexing means enabling a plurality of channels to be transmitted simultaneously over the line 3, said channels corresponding to different transmission wavelengths.

[0038] The signals they emit and receive are advantageously signals in the form of RZ pulses, and in particular solitons.

[0039] The line 3 comprises a plurality of optical fiber sections 4 with amplifiers 5 interposed between them.

[0040] In a manner that is entirely conventional for any transmission line, the line 3 includes means 6 interposed at each amplifier 5 for the purpose of measuring or deducing the power inlet to such an amplifier 5 and for returning information concerning said power to the emitter means 1, e.g. by using an auxiliary optical channel that corresponds to a wavelength different from those of the transmission channels.

[0041] Thus, the emitter means 1 are continuously aware of the power P_(ij) which is the power of channel i at the inlet to amplifier j, with this applying to i lying in the range 1 to n and j lying in the range 1 to m, where n is the number of transmission channels, and where m is the number of amplifiers 5.

[0042] The emitter means 1 include calculation means 7 which, for each channel i and as a function of the various powers P_(ij) measured for said channel i at the inlet of each of the m amplifiers, determine a power level P_(i1) which ought to be applied to the inlet of the first amplifier for said channel i. Said emitter means then adjust the power they emit for said channel i so as to cause it to converge on said value.

[0043] For example, the calculation means 7 determine for each channel i a value f_(i) which is a function of the powers (P_(ij))_(j=1,m).

[0044] They calculate the mean {overscore (f)} of the functions f_(i) and they relate said functions f_(i) to said mean.

[0045] They modify the inlet power reference by replacing P_(i0) with P′_(i0) such that:

[0046] P′_(i1)=ηG(f_(i))P_(i1) where η is such that ${\sum\limits_{i}P_{il}^{\prime}} = {\sum\quad P_{il}}$

[0047] where G is a predetermined function.

[0048] A particularly advantageous function f_(i) is: $f_{i} = {\sum\limits_{{j = 1},m}P_{ij}^{- 1}}$

[0049] More refined functions can be envisaged that take account of amplifier gain or variation in noise factor as a function of wavelength.

[0050] It will be observed that the values f_(i) can be calculated by the means 7 or can be obtained by any other means, and in particular by analog means using the signals transmitted over the auxiliary channel.

[0051] The function G advantageously corresponds to the function:

G(x)=x

[0052] Other functions can be envisaged, in particular for accelerating convergence.

[0053] The function G can also depend on in-line powers and can include a peak-limiting function to limit non-linear effects.

[0054] Another example of the function f_(i) which is advantageous when it is not possible to measure in-line powers or when it is desired not to interfere with the automatic gain equalization routines in-line, so as to avoid instabilities, is the following:

f _(i)=1/(P _(i1) ·P _(im))^(½)

[0055] It would also be observed that the above-described technique is particularly effective when the signal used is in RZ format, and in particular is in the form of solitons, since the non-linear limits are then offset towards high powers. 

1. A method of adjusting power for a WDM optical transmission system comprising emitter means, an optical transmission line, and receiver means, the method being characterized in that for each wavelength channel, the power emitted for said channel by the emitter means is adjusted as a function of the optical powers received for said channel at a plurality of points distributed along the transmission line.
 2. A method according to claim 1, characterized in that the various points distributed along the transmission line are the inlets to amplifiers distributed along said line.
 3. A method according to any preceding claim, characterized in that for each channel, a reference power is continuously determined as a function of the optical powers received for said channel at a plurality of points distributed along the transmission line, said reference power being the power that ought to be received for said channel at one of the points along the transmission line, and the emitter means are controlled so as to servo-control the power at said point on said reference power.
 4. A method according to claims 2 and 3 taken in combination, characterized in that said point is the inlet to the first amplifier.
 5. A method according to claim 3 or claim 4, characterized in that for each channel i, a parameter f_(i) is determined which is a function of the powers received at the various points of the transmission line, and in that the reference power is modified by replacing its preceding value with a reference value that is a function of said preceding value and also of the parameter f_(i).
 6. A method according to claim 5, characterized in that the new reference power is the product of the preceding reference power and a function of the parameter f_(i).
 7. A method according to claim 5, characterized in that the parameter f_(i) is related to the mean {overscore (f)} of the parameters f_(i) as determined for the various channels.
 8. A method according to any one of claims 5 to 7, characterized in that the parameter f_(i) is such that: $f_{i} = {\sum\limits_{{j = 1},m}P_{ij}^{- 1}}$

where P_(ij) represents the power at the jth point of the transmission line for channel i and where m is the number of points of said line.
 9. A method according to any one of claims 5 to 7, characterized in that the parameter f_(i) is such that: f _(i)=1/(P _(i1) ·P _(im))^(½)
 10. A method according to claims 6 and 7 taken in combination, characterized in that the new reference power is the product of the preceding reference value and a product η·f_(i).
 11. An optical transmission system, in particular using RZ type pulses, the system being characterized in that it includes means for implementing a method according to any preceding claim. 